1. Field of the Invention
This invention relates generally to an optical head device for recording information in an optical information recording medium and reproducing or erasing it therefrom, and more particularly, to an optical head device including an optical system which is free from any spherical aberration and can satisfactorily concentrate light.
2. Description of the Related Art
There has recently been a great deal of activity for realizing an optical head device for recording information in an optical disk and reproducing it therefrom by utilizing an optical system of maximum diffraction which is substantially free from any abberation.
The essential construction of such an optical head device is shown in FIGS. 7(a) to 7(c).
The device includes a semiconductor laser (LD) 1 defining a source of light, a condensing lens 2, a disk 3 having an information recording surface 4 provided with an information track 12, a beam splitter 10, a diffraction grating 14 and a cylindrical lens 15. A beam 7 of light leaving the LD 1 at a point 6 is concentrated by the lens 2 on the disk 3 at a point 5. The beam is also concentrated on a light detector 11 at a point 13.
Referring to the operation of the device, a diverging beam 7 of light is emitted by the LD 1 at the point 6 and converted by the condensing lens 2 to a converging beam. The converging beam passes through a transparent substrate forming the disk 3 and having a thickness d and is concentrated on the information recording surface 4 at the point 5.
The beam which has been reflected by the information recording surface 4 passes through the condensing lens 2, is separated from the beam 7 by the beam splitter 10 and is received by the light detector 11 whereby a light signal is produced.
The device has focus and tracking servo functions to ensure that the point 5 on the disk 3 on which the beam is concentrated by the condensing lens 2 is always located in an information track 12.
The focus and tracking functions of the device shown in FIGS. 7(a) to 7(c) are carried out by an astigmatic method and a twin spot method as will now be described briefly.
Referring first to the astigmatic method, an astigmatic device, such as the cylindrical lens 15, is provided in the path of the reflected beam for rendering it astigmatic. The position of the light detector 11 along the optical axis is adjusted to ensure that the beam radiated on the light detector 11 forms a minimum circle 13 of aberration if the focal point 5 is correctly positioned in the information track 12. The light detector comprises four detector elements 11a to 11d, as shown in FIG. 7(b).
If the optical disk 3 is displaced along the optical axis, the spots on the light detector 11 change their shape froma minimum circle of abberation as shown at 13 by a solid line to an elongated oval shape as shown by broken lines. This change is detected as an electrical focus error signal which is obtained by the calculation of a differential between the total output of one of the two pairs of diagonally disposed detector elements and the total output of the other pair, i.e., (11a+11c)-(11b+11d). This signal causes a focus actuator (not shown) to move the condensing lens 2 to correct any deviation of the focal point 5 from the information recording surface 4 along the optical axis.
According to the twin spot method, the beam 7 of the LD 1 is divided by the diffraction grating 14 disposed across its path into a plurality of beams, i.e., of the zero- and .+-.1-orders and is concentrated on the information track 12 as shown in FIG. 7(c). The beam of the zero-order is correctly radiated on the center of the information track and is used for signal reading and recording purposes. The beams of the .+-.1-order are slightly deviated from the track. The three spots lie in a line which is slightly inclined to the information track 12.
The diffracted beams of the .+-.1-order rays are received by the light detectors 11e and 11f and their difference (11e-11f) provides a tracking error signal which causes a tracking actuator not shown to move the condensing lens 2 to correct any deviation of the focal point 5 from the information track 12 in the plane of the information recording surface.
The optical head device as hereinabove described has a bit length and a track spacing which are so small as to enable the reading of any signal when the condensing system between the LD 1 and the condensing lens 2 is at a diffraction limit, so that the recording medium may be able to store a large amount of information at a high density.
Therefore, the converging beam which is incident to the information recording surface 4 must be free from any aberration so that the condensed spot 5 formed by the system at the diffraction limit may be radiated on the information track 12. The wave-front aberration which is permissible as the diffraction limit has a standard diviation of 0.07 .lambda. (Marechal limit), where, .lambda. is the wavelength of the LD.
It is known that when light is concentrated through a disk composed of a transparent substrate having a thickness d as hereinabove described, a wave-front aberration of formula (1) occurs as a spherical aberration of the fourth order: ##EQU1##
The concentration of light with substantially no aberration is required for an optical disk head. When the condensing lens 2 is designed, therefore, its spherical aberration is left without complete correction so that a balance may be obtained when its aberration W.sub.40 is offset by the aberration resulting from the passage of a beam through the substrate of the disk.
FIG. 8 is a view showing the optical condensing system in the optical head device of FIG. 7(a). In FIG. 8, l.sub.1 and l.sub.2 are the distance between the principal surface H of the condensing lens and the light emitting point 6 of the light source, and the distance between the principal surface H' of the lens and the light concentrating point 5 on the disk 3, respectively. When the optical head is designed, the distances l.sub.1 and l.sub.2 are selected to position the light emitting point 6 and the light concentrating point 5 in a conjugate relation with respect to paraxial rays.
In other words, the distances have the following relation to the focal length f.sub.0 of the condensing lens 2: ##EQU2##
The parameters (e.g., shape and thickness) of the condensing lens 2 are selected as minimize any disorder in the wave front of the beam to be concentrated, or any wave-front aberration when the positional relation satisfying formula (2) exists.
The optical head device which has hereinabove been described, however, include various factors that give rise to wave-front aberration and thereby lower its recording and reproducing performance. Spherical aberration, which is one of the components of wave-front aberration, will hereinafter be discussed.
Spherical aberration is due to (1) an error in thickness of the disk, (2) variation in the refractive index of the disk, (3) deviation of the shape of the refracting surface of the condensing lens from a designed one, (4) error in thickness of the condensing lens, (5) variation in the refrative index of the condensing lens, etc.
For example, if the disk comprises a polycarbonate substrate having a refractive index N of 1.55 and if the condensing lens has a numerical aperture (NA) expressed as sin.theta..sub.2 =0.5, formula (1) gives the rms wave-front aberration of 0.014 .lambda. for light having a wavelength of 0.78 .mu.m if the disk has an error of 50 .mu.m in thickness. This amounts to 20% of the allowable rms wave-front aberration of 0.07 .lambda. which has hereinabove been stated, though the error in thickness of the disk is very small.
All of the other factors stated at (2) through (5) above also give rise to the fourth-order spherical aberration which is symmetric with respect to the optical axis. The superposition of each factor also gives rise to the component of aberration which is symmetric with respect to the optical axis and as a result causes the fourth-order spherical aberration to remain. The wave-front aberration can be expressed as W.sub.40 .multidot..rho..sub.4, where .rho. is the normalized radius of the pupil which is greater than 0 and less than 1.
It is well known that the presence of any such spherical aberration results in a lower central intensity of the light concentrating point (spot) 5 and thereby a larger diameter thereof. The increase in diameter of the light concentrating point 5 leads to a reduction in OTF of the optical system resulting in a reduction in the recording and reproducing density of the optical disk head and its performance.